Method of curing loss of circulation of a fluid used in drilling a hole in an underground formation



Dec. 10, 1968 F. H. MEIJS 3,415,31

METHOD OF CURING LOSS OF CIRCULATION OF A FLUID USED IN DRILLING A HOLE IN AN UNDERGROUND FORMATION Filed June 28, 1967 INVENTORI FRANCISCUS H. MEIJS BY: l

IS AGENT United States Patent 3,415,318 METHOD OF CURING LOSS OF CIRCULATION OF A FLUID USED IN DRILLING A HOLE IN AN UNDERGROUND FORMATION Franciscus H. Meijs, Port Harcourt, Nigeria, assignor to Shell Oil Company, New York, N.Y., a corporation of Delaware Filed June 28, 1967, Ser. No. 649,533 Claims priority, application Netherlands, July 20, 1966, 6610183 9 'Claims. (Cl. 166-29) ABSTRACT OF THE DISCLOSURE A method of preventing loss of circulating fluid during drilling of formations containing large cavities comprising injecting into such cavities a viscous liquid containing suspended particles and a viscosity breaking liquid.

The invention relates to a method of curing loss of circulation of a fluid used in drilling a hole in an underground formation. During the drilling of such holes, very permeable, cavernous or fractured rocks may be passed by the hole, and the interior of the hole may come into communication with largevolume spaces such as cavity system or cavity systems within the rock, with the result that the drilling fluid, which is circulated through the hole, is drained from the hole into these spaces. If the loss of drilling fluid into these spaces is not made up by the supply of new drilling fluid to the top of the hole, the fluid level in the hole will descend and may give rise to serious difiiculties or may lead to abandonment of the hole.

Background of the invention The loss of fluid from a hole being drilled, however, can often be combated by adding plugging material to the fluid circulating through the hole. Such plugging material (e.g., fibers, flakes, sawdust, shredded wood) has, preferably, such a size that when the fluid passes out of the hole into a cavity system in the formation, the plugging material is filtered out in the openings existing between the hole and the cavity system. The filtered-out material plugs these openings, thereby decreasing the passage therethrough to such an extent that the plastering material which is normally present in the drilling fluid seals the passage through the pores left between the plugging material.

Another way to obstruct the passage between the hole and a cavity system in the formation being drilled, is by injecting a substance (such as cement), which substance sets into a solid mass.

However, the above methods fail when used in an attempt to cure loss of circulation when the holes are in communication with cavity systems via large-sized openings.

It has been proposed in copending application Ser. No. 406,259, filed Oct. 26, 1964, now Patent No. 3,347,316 to inject into a cavity system communicating with a borehole being drilled, a suspension of particles in a liquid, which suspension has the property that the particles readily separate out by gravity. Because of this property, the particles are deposited in the cavity system at those spots where the velocity of the injected fluid has dropped to a sufliciently low value. The amount of particles deposited within the cavity system gradually grows at such spots and fills up locally the passage through the cavity system. On the side of the mass of particles facing the borehole, there is subsequently deposited a sealing sheath by injecting a fluid having plastering properties. This method of blocking the passage through which the loss of drilling fluid occurs, by building a dam of particles in the cavity 3,415,318 Patented Dec. 10, 1968 system, is a very suitable way of dealing with fluid losses taking place via openings in the wall of the borehole which are of such a size that they cannot be closed off by plugging miaterials.

It is an object of the present invention to close the passage through the cavity system. within a shorter period than with this known method. Another object is moreover, that the dam built in the cavity system for closing off the passage therethrough is able to withstand higher pressure ditferences thereacross than obtainable by the prior art method. Still another object is to carry out the new method of plugging cavities Without pulling the bit from the hole.

Swmlmary of the invention According to the invention, a method of curing loss of circulation of a fluid used in drilling a hole in an underground formation, which hole communicates with at least one cavity system, comprises the following steps:

Preparing a suspension of particles in a viscous carrier fluid;

Injecting this suspension into the borehole simultaneously with but separately therefrom a liquid having viscosity-breaking properties;

Mixing the suspension and the viscosity-breaking liquid just before entering the cavity system, whereby the viscosity of the carrier fluid is broken just after the suspension has entered the cavity system;

Separating the particles within the cavity system from the low-viscosity carrier fluid by gravity;

Stopping the injection of the suspension into the cavity system after the passage through the cavity system has been blocked; and,

Injecting into the cavity system a fluid having a plastering properties.

Brief description of the drawing The invention may be carried out into practice in various ways, but one specific embodiment will now be described with reference to the accompanying drawing, in which FIGURES 1, 2, 3 and 4 schematically show sequential steps by which a cavity system can be plugged off by means of the method according to the invention.

In FIGURE 1 there is shown a vertical section through a borehole i1 passing through a permeable formation 2. During the drilling of this hole, a drilling fluid is circulated therethrough, so as to raise the cuttings to the surface and to prevent influx of formation fluids into the hole. So as to prevent the loss of drilling fluid into the pore space of the formation 2, which in the example as given is of a permeable nature, the drilling fluid used in the hole 1 has plastering properties. A plastering material, such as finely divided clay, may thereto have been added to the fluid, which material when the fluid passes into the pore space of the formation 2, is filtered from the fluid and left on the wall of the borehole 1 in the form of a plastering cake 3. Since the resistance to flow through the plastering cake is rather high, the further flow of fluid out of the borehole 1 into the pore space of the formation 2 is decreased and nearly the full pressure difference existing between the interior of the borehole and the pore space of the forma tion 2 prevails over this cake 3.

However, the drill bit (not shown) on its way downwards into the formation 2 has passed a cavity 4 which forms part of a (not shown) cavity system of large volume and extending over a large area.

The drilling fluid which is being circulated through the borehole 1 then passes out of the borehole 1 into the cavity 4 of which only part has been shown in the drawing. As the entrance 5 to the cavity 4 has dimensions which are a great many times larger than the dimensions of the plastering material present in the drilling fluid, this material is not able to seal this entrance to prevent the fluid from entering the cavity.

As the cavity system is of very large size, the permeable area of the formation 2 bounding the cavity system is also of a very large size, which makes sealing thereof by the plastering material present in the fluid flowing into the cavity 4 impossible.

When the large loss of fluid into the cavity 4 has become apparent, the drill string (not shown) is retracted from the hole and the drill bit removed therefrom, whereafter the drill string 6 (or any other type of pipe) is lowered into the hole 1 (see FIGURE 2) until the lower end of the string 6 is located at some distance above the entrance to the cavity 4.

The entrance 5 to the cavity 4 will normally be found at or near the bottom of the hole 1. Methods exist, however, of determining the exact position of this entrance, e.g., the spinner survey method or the temperature survey method. The spinner survey method makes use of a flow meter (spinner) which is moved through the hole 1. This flow meter will show a certain flow velocity of the fluid above the entrance 5 and a flow velocity of zero or nearly zero below the entrance 5. The temperature survey is carried out by measuring the temperature of the fluid at various levels in the borehole. The nonflowing fluid present in the hole at a level below the entrance 5 will have adopted the formation temperature, whereas the liquid present above the entrance and flowing towards this entrance will have a temperature different therefrom.

Once the lower end of the pipe 6 is positioned at a level above the entrance 5, the top of the annular space 7 surrounding the pipe 6 is brought into communication with a supply of a suspension of particles in a viscous carrier fluid. If desired, the suspension to be injected can be made before the pressure thereof is raised to the injection pressure. In another way the viscous fluid may first be pressurized to the desired value whereafter the particles are added thereto.

The particles may be formed, e.g., by quartz, barite or iron ores, such as limenite or magnetite. These particles should have a specific gravity greater than the specific gravity of the mixture of carrier fluid and the viscosity breaking liquid.

The suspension is pumped down through the annulus 7 of the hole, and the flow thereof is, due to the pressure difference existing between the hole 1 and the cavity 4, directed into the cavity 4 (see FIGURE 2). The injec tion pressure should be well below fracturing pressure to prevent fracturing of the formation. Simultaneously with the suspension, a viscosity breaking liquid is pumped down through the pipe 6, which liquid intermingles with the suspension on leaving the lower end of the pipe 6. This mixing can be improved by slowly rotating the pipe 6 and/or by moving the pipe 6 alternately upwards and downwards in the hole.

Description of preferred embodiments The viscosity-breaking liquid is chosen such, that on being mixed with the suspension, it rapidly influences the viscosity of the carrier fluid of the suspension. On entering the cavity 4 via the opening 5, the viscosity of the carrier fluid quickly drops to such a low value that the particles readily separate from the carrier liquid by gravity. These particles are deposited in the cavity 4, until an increase of the injection pressure indicates that the passage through the cavity 4 has been closed off by a mass of particles 8 (see FIGURE 3). The injection of the suspension, as well as of the viscosity-breaking fluid, is then stopped. Thereafter a fluid having plastering properties is injected into the top of the pipe 6. This fluid drives the viscosity-breaking liquid out of the string 6 and into the cavity 4, where it is passed through the pores of the dam 8. Thereafter the plastering fluid passes out of the string 6 and enters the cavity 4, forming a plastering cake 9 on the 'wall of the dam 8 and increasing the thickness of the cake which might already have been deposited on the wall of the formation 2. As the cake 9 forms a continuation of the mud cake 3 which partly was already present on the wall of the borehole 1 and partly was deposited later on, the communication between the borehole and the cavity system as well as the formation pore space is broken and no further fluid will escape from the borehole. If the formation 2 has an impermeable nature, this plastering cake will only be deposited on the side of the mass 8 facing the borehole 1.

A rise in pressure at the injection point of this plastering fluid will signal the formation of the cake 9. Thereafter the top of the annular space is opened and the suspension present in this annulus is circulated out of the hole by the plastering fluid or by the drilling fluid.

After the injection of the plastering fluid into the hole 1 and part of the cavity 4, the pressure in the hole may be increased so as to test the strength of the obstruction 8 in the cavity 4 against a difference in pressure existing over said obstruction. If the obstruction does not hold, the whole process can be carried out anew. If desired, a plastering fluid may be used which hardens 'after some time. Thus, cement may be applied which strengthens the dam by the formation of a strong solid impermeable layer thereupon.

In a preferred manner, slugs of plastering fluid are injected between slugs of suspension and/or viscosity breaking liquid. Hereby, the closing of the dam will be signalled by an abrupt increase in injection pressure, which results from the plastering of the dam by one of the slugs of plastering fluid. The slugs of plastering fluid may be injected via the pipe 6 and/or via the annular space 7. The injection of suspension into the hole is interrupted during the injection of the plastering fluid into the cavity, if this fluid is injected through the pipe 6 only.

The viscous carrier fluid in which the particles are suspended may be formed by a gel or an emulsion.

When applying a gel for carrying the particles down the hole, use may be made, e.g., of water to which guar gum is added as a gelling agent. An aqueous solution of hydrochloric acid is then applied as viscosity-breaking liquid. When applying an emulsion, it is preferred to use a water-in-oil emulsion. When using an emulsion of by volume of water in l9l9.5% volume of gas oil stabilized with l0.5% by volume of an emulsifier, such as Dowell Emulsifier M41, up to 1.5 kilograms of coarse sand or gravel can be suspended per liter of the emulsion. At ambient temperature, this emulsion is stable for at least one day, and at a temperature of 80 C. the emulsion is stable for three hours. Many of the deemulsifiers used for crude oil emulsion dehydration and marketed by firms like Servo and Tret-O-Lite will cause rapid breaking of the emulsion when applied in a concentration of 500 ppm. or less.

The particles suspended in the viscous carrier fluid may range in size from sand to gravel. Since in the prior art method the particles were suspended in a carrier liquid which is of lower viscosity than the carrier fluid as applied in the present method, the particles used in the present method may under corresponding conditions be of larger size and/or greater specific gravity than in the prior art method. The application of this larger-sized or heavier particles, which is allowed by the use of a viscous carrier fluid, will give a stronger dam in the cavity. Preferably, the operation is started with the injection of largesized particles (such as gravel) and finished by substituting these particles either gradually or abruptly by smallersized particles (such as sand), which smaller-sized particles will fill up the openings leading to the pore space of the large-sized particles forming the dam, whereby a better sealing will be obtained during the subsequent injection of the plastering fluid. If desired, plugging agents may be added to the plastering fluid.

In another manner, a mixture of gravel and sand can be suspended in the carrier fluid. Then the interstices between the gravel building up the dam during separation of the particles of the fluid in the cavity will be filled up by the sand particles. However, the sizes of the particles may not differ too widely, since otherwise the coarse and the fine particles will be deposited at separate spots.

As angular particles will, when settling, form a stronger barrier or obstruction than round particles, the use of the first type of particles is to be preferred when applying the method according to the invention.

It will be clear that the introduction of the viscositybreaking liquid may also take place via the drill string to which a drill bit is attached, provided that the openings in the bit have a sufiiciently great area. Slowly rotating the bit and/ or moving the string alternately upwards and downwards will prevent clogging of the passages between the bit and the borehole wall by the suspension flowing past the bit. These movements will also improve the mixing between the suspension and the viscosity-breaking liquid.

Application of a check valve in the passage through the drill string is recommended to prevent inadvertent entry of the suspension into the drill string, since this might lead to clogging of the passage through the string.

The application of a viscous carrier fluid to transport the particles from the surface to the cavity system which is to be closed off, has the advantage over the known method, that per volume of carrier fluid a larger amount of particles can be transported. Thus, the present method will allow quicker closing of a cavity system than the known method, in which a carrier fluid from which the particles readily separate is being used.

Moreover, the viscous carrier fluid allows the use of heavier and/ or coarser particles which results in the formation of a closure within the cavity which Will be able to withstand higher pressure difierences thereacross than the closure formed by the old method.

Although the present invention has been illustrated by a description thereof when applied for closing the passage through cavity systems located in permeable formations, it is not limited thereto. With equally favorable results the invention may be applied if the cavity system to be closed off is located in an impermeable formation.

It will further be clear that the present invention is not limited to' the examples given above, but that without departing from the spirit of the invention various types of particles, various types of fluids for suspending the particles therein, and various types of plastering fluids as well as of viscosity-breaking liquids can be used for carrying out the method according to the invention.

I claim as my invention:

1. A method of curing loss of circulation of a fluid used in drilling a hole in an underground formation which hole communicates with at least one cavity system, comprising the following steps:

preparing a suspension of particles in a viscous carrier injecting this suspension into the borehole simultaneously with but separately from a liquid having viscositybreaking properties; mixing the suspension and the viscosity-breaking liquid just before entering the cavity system, whereby the viscosity of the carrier fluid is broken just after the suspension has entered the cavity system;

separating the particles within the cavity system from the low-viscosity carrier fluid by gravity;

stopping the injection of the suspension into the cavity system after the passage through the cavity system has been blocked; and,

injecting into the cavity system a fluid having plastering properties.

2. A method according to claim 1, wherein the size of the particles is decreased during the injection of the suspension into the hole.

3. A method according to claim 1, wherein the viscous carrier fluid is a gel.

4. A method according to claim 3, wherein the carrier fluid is a mixture of guar gum and water.

5. A method according to claim 4, wherein the viscositybreaking liquid is an aqueous solution of hydrochloric acid.

6. A method according to claim 1, wherein the viscous carrier fluid is an emulsion.

7. A method according to claim 6, wherein the emulsion is a water-in-oil emulsion.

8. A method of claim 1 wherein the viscosity-breaking liquid is injected in slugs alternated by the injection of slugs of the plastering fluid.

9. A method of claim 1 wherein the suspension is injected in slugs alternated by the injection of slugs of the plastering fluid.

References Cited UNITED STATES PATENTS 3,280,912 10/1966 Sheflield 166-29 3,347,316 10/1967 Havenaar 16629 3,368,636 2/1968 Schmidt et al -72 JAMES A. LEPPINK, Primary Examiner.

U.S. Cl. X.R. 

